Added parallax scrolling background

Hello Hakim,

Reveal.js is awesome! I just added a parallax scrolling effect to the presentation background and would like to share the changes I made to the code with you.

I added two parameters to the config object (parallaxBackgroundImage and parallaxBackgroundSize) and updated README.md accordingly.

The background image used is made by Eli Mergel and is published under Creative Commons license. I found it on Flickr: https://secure.flickr.com/photos/sp1te/3436256585/sizes/o/in/pool-856427@N25/.


Best regards,
Michał